To downgrade to Firefox 3.6 first uninstall Firefox 4, but do not select the option to "Remove my Firefox personal data". If you select that option it will delete your bookmarks, passwords and other user data.

You can then install the latest version of Firefox 3.6 (version 3.6.17) available from http://www.mozilla.com/en-US/firefox/all-older.html - it will automatically use your current bookmarks, passwords etc.

To avoid possible problems with downgrading, I recommend going to your profile folder and deleting the following files if they exist - extensions.cache, extensions.rdf, extensions.ini, extensions.sqlite and localstore.rdf. Deleting these files will force Firefox to rebuild the list of installed extensions, checking their compatibility, and reset toolbar customizations.

For details of how to find your profile folder see https://support.mozilla.com/kb/Profiles

Tony E,

Thanks so much. That's the perfect solution. I had all the files you recommended I delete except the .cache file. I did not check the box for removing data during the uninstall of version 4 and when I installed 3.6.17, all the bookmarks were restored, all the extensions were rebuilt and it even opened the exact page I had left open in the one tab I had not closed in 4.

In short, the only thing I could possibly suggest to improve your solution would be to move the part about the files to delete up to the first sentence as I uninstalled version 4 before I went to delete the files, and I think it would be a little easier for the average user to find these files if they look for the Profile folder they're in BEFORE they uninstall Firefox.

Thanks again for your detailed and accurate help!